The present invention relates to a semiconductor device and, more particularly, to a semiconductor device which is operable at ultrahigh frequency.
An ultrahigh-mobility FET having an aperating range which exceeds conventional field effect transistors (hereinafter abbreviated as FET) has already been proposed by H. Sakaki et al. and discussed in Japanese Journal of Applied Physics Vol. 19 (1980), p.94. According to their discussion when the width of the channel layer of a MOSFET is sufficiently reduced so that the channel layer meets the conditions of being a quantum well thin line in which electrons which are confined within this channel have freedom to move effectively only in the channel direction, the probability of electrons within the channel being scattered becomes extremely little, so that the mobility of electrons reaches 10.sup.7 to 10.sup.8 cm.sup.2 / V.s.
However, the device structure proposed by them involves difficulties in producing a device, in which the probability of electrons being scattered at the side surface of the channel is low , with the present microfabrication technology, and therefore it is difficult to realize the proposed device structure.
The following fact has heretofore been known the physical properties of a semiconductor device wherein two kinds of semiconductor layer which are different from each other in terms of electron affinity x are stacked to form a superlattice structure.
Let us consider a structure in which two semiconductor layers 1 and 2 having a different electron affinity x are stacked in the form of a superlattice as shown in FIG. 2A.
Representing the respective electron affinities x and thicknesses of the semiconductor layers 1 and 2 by x.sub.1, x.sub.2 and a, b, respectively, the effective electrostatic potential felt by conduction electrons within the alternating semiconductor layers is such as that shown in FIG. 2B. More specifically, this potential is a periodic potential having a depth V.sub.l D=x.sub.1 -x.sub.2 in the direction of its thickness (the direction of the arrow A). In FIG. 2B, a and b denote the respective thicknesses of the semiconductor layers 1 and 2, while (I) and (II) denote channel and barrier regions, respectively. It is well known, as discussed by R. de L. Kronig and W. G. Penny, that the relationship between the electron energy E and the wave number k of electrons has a structure such as that schematically shown in FIG. 3 (see Proceedings of Royal Society of London, Vol. A130 (1931), R. de L. Kronig and W. G. Penny, p.499).
It will be clear from FIG. 3 that points such as those indicated by A, A', B and B' in the figure are points of inflexion of the E-k curve (the relationship between the wave number k and momentum P is expressed using h=h/2.pi., where h is Planck's constant, as follows: P=nk). On the other hand, as described in solid state physics textbooks, for example, Handbook of Solid State Physics Vol. 1, C. Kittel (translated by Uno, Tsuya and Yamashita), Maruzen Kabushiki Kaisha, 1981, p.205, the effective mass m.sup.* of electrons within a solid body is given by the following equation using the E-k curve: EQU m.sup.* /1=n.sup.2 /1 .delta..sup.2 k.sup.2 /.delta..sup.2 E
It will be understood from the above equation that the effective mass m.sup.* of electrons becomes exceedingly large near points of inflexions such as those indicted by A, A', B and B' in FIG. 3 and becomes infinite at these points. More specifically, if the channel layer I and the barrier layer II are alternately and repeatedly stacked to form a periodic structure, the effective mass of electrons increases in the direction of the arrow A in FIG. 2B.
Although the movement of electrons in the direction of thickness of alternating semiconductor layers has been discussed above, it should be noted here that the twodimensional movement of electrons in the direction of plane of the stacked structure is equivalent to that of free electrons.
Although the physical properties of the abovedescribed superlattice semiconductor layers has already been known, the application thereof to semiconductor devices has not yet been realized due to underdevelopment of peripheral technology.